Monitoring of digital transmission systems

Abstract

An arrangement for monitoring slip in a digital transmission system. A PRBS sequence is transmitted in one channel or timeslot of the system. At a receiving station a characteristic group of N bits of the received sequence is assembled. The assembled group is compared with locally formed groups of N bits which would be expected to occur if a slip has occurred.

I claim: 
     
       1. Apparatus for monitoring slip in a digital transmission system or the like that receives digital data, stores it in a memory and outputs it in synchronism with a local clock signal, the apparatus comprising: memory means for storing receiving digital data;   first means, coupled to the memory means and responsive to the received digital data stored therein, for accumulating and storing a first group of N successive bits of a first PRBS sequence that is part of the received digital data stored in the memory means, N being an integer that is sufficiently large that N successive bits characterize the PRBS sequence, and also for producing at a first output signals indicative of the first group of N successive bits;   second means, coupled to the memory means and responsive to the received digital data stored therein, for generating a second PRBS sequence at least at one time identical to the first PRBS sequence and also for producing from the second PRBS sequence at second, third and fourth outputs, signals indicative of second, third and fourth groups of N successive bits, the second, third and fourth groups respectively indicative of a negative slip, no slip and a positive slip; and   comparison means coupled to the first, second, third and fourth outputs, for comparing the first group N successive bits with each of the second, third and fourth groups of N successive bits.   
     
     
       2. Apparatus as in claim 1 wherein the digital transmission system divides the received digital data into a plurality of n-bit bit time slots each of which periodically occurrs in sequence, n being an integer less than or equal to N, and wherein the first group of N successive bits is accumulated from N-31 n bits of a first time slot and n bits of the next succeeding time slot. 
     
     
       3. Apparatus as in claim 1 or 2 wherein the first means and the second means comprise a processing means for executing a stored program. 
     
     
       4. Apparatus as in claim 3 wherein the processing means includes look-up tables for generating the second, third and fourth groups of N successive bits. 
     
     
       5. A method of monitoring slip in a digital transmission system or the like that receives digital data, stores it in a memory and outputs it in synchronism with a local clock signal, the method comprising the steps of: receiving a PRBS sequence transmitted with the digital data;   accumulating from the received PRBS sequence a characteristic group of N successive bits;   generating a local PRBS sequence initially in agreement with the received PRBS sequence;   forming groups of N successive bits of the local PRBS sequence that would be expected to arise from the accumulating step in the absence of a slip and in the event of a slip; and   comparing the group of N successive bits of the accumulating step with each of the groups of N successive bits formed by the forming step.